In order to obtain the polymerization or cross-linking reaction in polymerizable resins, use is conventionally made of a thermal method.
This method exhibits significant disadvantages, especially on account of the thermal gradients and thus of the thermal stresses induced by the treatment of the material. Moreover, the need to arrange the manufactured article in a suitable oven limits the dimensions of the manufactured article which can be obtained using this method.
For the purpose of overcoming the disadvantages of the thermal processes, there has recently been proposed the use of electron beams for the polymerization of resins which exhibit high levels of reactivity to the formation of cross-linking bonds by means of irradiation using electron beams. This technique has already been extensively investigated experimentally in the field of the production of protective coatings of paper or wood and in the treatment of inks, lacquers and the like, and moreover is the subject of investigation in the polymerization of the resin matrix for the production of composite materials.
The process of polymerization by irradiation using electron beams exhibits significant advantages as compared with the conventional thermal methods. In fact, the technique employing electron beams permits the polymerization to be achieved in very short periods of time and without inducing structural deformations due to thermal effects, as does, however, happen in the processes of polymerization by heating.
Moreover--in contrast to the thermal processes, in which the entire structure subjected to polymerization is treated in a homogeneous manner--the technique involving irradiation using electron beams permits the localization of the treatment, very precisely focusing the electron beam selectively in the desired zone. The possibility of using the electron beam to "brush" the surface of the structure to be polymerized permits an enhanced ease of treatment of parts of large dimensions.
Notwithstanding the aforementioned advantages, the technology involving electron beams exhibits a considerable limit due to the high beam energy required to pass through the thickness of the material to be polymerized. The thickness which can be penetrated by the electron beams is essentially dependent upon the energy of the beam and upon the density of the material. Using z (a quantity referred to as the "specific thickness" or "surface mass") to designate the product of the density of a material (in g/cm.sup.3) and the thickness of said material (in cm), the relation between the energy V.sub.B of the electron beam (expressed in eV) and the specific thickness z penetrated by the electron beam is approximately given by: EQU z(g/cm.sup.2)=5.1.times.10.sup.-7 V.sub.B -0.26
This relation is valid for beam energies equal to or greater than 1 MeV.
Using a 10 MeV accelerator (this being a considerable energy for this type of application, as the electron accelerators normally employed for industrial applications do not exceed 5 MeV), the specific thickness (z) which can be treated is approximately 4.8 g/cm.sup.2. By way of example, assuming a density of the resin of 1 g/cm.sup.3, the thickness which can be treated is approximately 4.8 cm. On the other hand, in the case of a high density, for example 4.8 g/cm.sup.3,the thickness which can be treated is reduced to 1 cm.
The problem of the limitation with regard to the thickness which can be treated using electron beams arises in particular in the case of composite-material structures, the specific thickness z of which may locally exceed 4-5 g/cm.sup.2.
A method was recently proposed (U.S. Pat. No. 4,789,505) which combines polymerization using electron beams (for specific thickness values below 4 g/cm.sup.2) with polymerization using X-rays (for greater thicknesses) which are obtained by interposing a metal target between the electron generator and the material to be treated. The electron beam strikes the metal target, generating X-rays. The interaction between the X-rays and the resin initiates the polymerization reaction by means of the ionization induced by the release of energy within the matrix. This method exhibits numerous disadvantages. In fact, the generation of X-rays as a result of the bombardment of the target by the electron beam is a process which has a low yield, so that it becomes necessary to use generators of high energy (in the order of 10 MeV) and considerable power (at least 10 kW) in order to generate X-rays of sufficient intensity. This involves significant equipment costs and necessitates the adoption of pertinent radio protection measures.
Moreover, the X-rays employed for the polymerization cannot be focused onto a particular area of the matrix, and accordingly the advantage of treating selected zones of the material is lacking.